Bonding of Plastics

Plastics are bonded to one another or to other materials. They differ widely in their affinity for bonding. Some plastics are easy to bond (e. g., PVC-U, ABS, PS), whereas others can be bonded only after special preparation (e. g., PTFE). On account of the low polarity and surface tension of certain plastics (e. g., polyolefins) there are still no adhesives for pipe joints which are durably resistant to shear strain and pressure. The choice of adhesive is determined by the substrate and the mechanical, thermal, and chemical requirements [69] — [75].

Poly(Vinyl Chloride). Rigid PVC (PVC-U) and chlorinated PVC (PVC-C) pipes, sheets, and films are used worldwide in the construction industry. They are joined by adhesives consisting of PVC solutions in a mixture of solvents, which operate by the principle of diffusion bonding. The bond is established by diffusion of the adhesive into the surfaces and the temporary dissolution of the PVC (cold swell welding).

For the bonding of uncalibrated pipe joints, adhesives with gap-filling properties are used, in which PVC of a certain molecular mass is dissolved in a solvent consisting predominantly of THF, cyclohexanone, or IV-methylpyrrolidone. The viscosity range is 2000 — 3500 mPa s. The application time of the adhesive ranges from 1 to 5 min. The adhesives are preferably made thixotropic so that they do not run off the surfaces to be joined after application and to avoid the damage to the plastic material because of accumulation of adhesive (the excess solvent can cause tensile corrosion). After pre­liminary cleaning of pipe and fitting with solvent (cleaning and dissolution) and application of the adhesive to both sides, the pipe and the fitting immediately are fitted together without turning. The bond is established by diffusion of the adhesive into the surfaces and temporary dissolution of the PVC (diffusion bonding).

PVC adhesives, usually consisting of solutions of post-chlorinated PVC in methylene chloride, are used in the case of inorganic acids as flow-through media in PVC pipe because of their better chemical resistance. Since PVC adhesives have no gap-filling properties a special application technique is required. This priming method is little used today.

Contact adhesives based on polychlorobutadiene or polyurethane are used for large — area bonds between PVC-U and PVC-C sheets and wood or metal.

Vessels are often lined with thin PVC-U and PVC-C films (1-2 mm thick). Contact adhesives are used with usual application techniques. For bonding cooling tower films, special adhesive solutions, which are harmonized in viscosity and thixotropy with the processing techniques (roll or pot application), analogous to PVC pipe adhesives, are used.

Adhesives for bonding plasticized (flexible) PVC (PVC-P) films are adhesive solutions based on THF (80 — 90 %) and PVC-P (10 — 20 %). Adhesives based on nitrile rubber or polyurethane in a plasticizer-resistant formulation for the bonding of PVC-P films made from differently formulated compounds are also suitable.

Подпись: Applications of AdhesivesAdhesives based on polymethacrylates and vinyl copolymers are used for the con­tinuous bonding of PVC-P films to metal sheets (coil coating). In this process, the adhesive is applied by machine to the cleaned sheets, the solvent is removed, and the film subsequently is rolled into the bed of adhesive heated to 140 -180 °С.

Polystyrene, Styrene-Acrylonitrile Copolymers, and Styrene-Butadiene Copolymers. These plastics may be bonded to themselves or to one another simply by treating the surfaces to be joined with pure solvents, such as dichloromethane, ethyl acetate, or 2-butanone. However, this method of bonding, which also is known as solvent welding, does involve the risk of material corrosion. Therefore, it is advisable to use solutions of polymer raw materials in mixtures of organic solvents with a low risk of material corrosion. Depending on the field of application, especially in combining different materials, contact and nitrile-rubber adhesives containing aliphatic hydrocar­bons as solvents also may be used. Pipes of ABS/ASA (ASA: acrylonitrile — styrene — acrylic ester polymer) for high-temperature wastewater run-off systems (max. 90 °С) preferably are joined with solutions of ABS/ASA in acetone or 2-butanone, although they also may be bonded with THF-PVC systems. The solutions are of medium viscosity (max. 2000 mPa s) and have a predominantly thixotropic rheology.

Solutions of ABS in 2-butanone are preferred for use in pressure applications, such as compressed-air conveying systems. As with PVC, the bond is established by diffusion.

Rigid polystyrene foam is used for the manufacture of composite sheets, for example, for containers and prefabricated building boards. The rigid foam is combined with metals, wood-based materials, asbestos, cement, and plastic sheets. Polyurethane ad­hesives are particularly effective in this field.

Polyolefins. Polyethylene, polypropylene, and polybutene can be bonded only after treatment to increase the surface energy, generally by oxidation, and make the surfaces receptive to bonding. Pretreatment can be carried out with an oxidizing flame, with oxidizing chemicals, or by electrical discharge.

Nowadays, the flaming process is rarely used: in the lining of containers with polyethylene and in the printing of bottles (Kreidel process). Among the chemical pretreatments, immersion in chromosulfuric acid (for a few seconds to a few minutes), followed by careful rinsing with water has proved successful. Thin plastic layers for the production of laminated films for use as packaging materials are best pretreated by corona discharge.

The pretreated polyolefins can be bonded with contact adhesives, epoxy resin adhesives, one — and two-component polyurethane adhesives, and hot-melt adhesives.

Polyamides are bonded with concentrated (ca. 85 %) formic acid. Two-component polyurethane or epoxy resin adhesives are suitable for bonding to metals, for example, for holding bearing bushings in place.

Подпись: AdhesivesPolymethacrylates can be bonded by treating the surfaces to be joined with a mixture of dichloromethane and dichloroethylene. However, this does involve the risk of material corrosion. In such cases, polymerization adhesives cured by light can be used. Polyurethane, epoxy, and contact adhesives are also suitable.

Fiberglass-Reinforced Plastics. To obtain high-quality pipe connections, the sur­faces to be joined must be roughened and the pipes carefully tapered. Polyester adhesives or thermosetting epoxy resin adhesives are best suited for bonding. A pressure of 50-100 N/cm2 is applied to the joined surfaces.

Fluoropolymers. There are no known adhesives for fluoropolymers that give bonds of adequate strength without pretreatment. Simple, low-quality bonds can be obtained with contact adhesives. After chemical pretreatment, for example, with a solution of an alkali metal naphthalenide in THF, polyfluorocarbons can be bonded with high strength by using epoxy and polyurethane adhesives.

Curable Molding Compounds. High-strength bonds involving these plastics are obtained with epoxy resin adhesives. The adherents must be roughened lightly and thoroughly cleaned beforehand. Laminates, including decorative laminates, of phenolic resin — or melamine resin-impregnated papers or fabrics can be bonded with contact adhesives.

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